Stall control for vane axial compressors

ABSTRACT

IMPROVED STABILITY WITH RESPECT TO AERODYNAMIC STALLING IN AN AXIAL FLOW GAS COMPRESSOR IS OBTAINED BY APERATURING ONE OR MORE BLADES, THE APERATURED BLADES TYPICALLY FORMING A PLURALITY OF BLADE GROUPS ALTERNATING WITH GROUPS OF UNAPERATURED BLADES.

y 23, 1972 R. E. SALISBURY ETAL 3,664,757

STALL CONTROL FOR VANE AXIAL COMPRESSORS Original Filed July 18, 1969 7fm ressure F/ow , mum

United States Patent 3,664,757 STALL CONTROL FOR VANE AXIAL COMPRESSORS Robert E. Salisbury, Whittier, and John W. Erickson, Huntington Beach, Calif., assignors to Preco, Inc., Los Angeles, Calif.

Original application July 18, 1969, Ser. No. 842,983, now Patent No. 3,597,111, dated Aug. 3, 1971. Divided and this application Apr. 7, 1971, Ser. No. 132,149

Int. Cl. F04d 5/00 US. Cl. 41553 3 Claims ABSTRACT OF THE DISCLOSURE Improved stability with respect to aerodynamic stalling in an axial flow gas compressor is obtained by aperturing one or more blades, the apertured blades typically forming a plurality of blade groups alternating with groups of unapertured blades.

This application is a division of application Ser. No. 842,983, filed July 18, 1969 under the title Blade Mount and Stall Control for Vane Axial Compressors, now Pat. No. 3,597,111, issued on Aug. 3, 1911.

This invention relates generally to axial flow fans, blowers and gas compressors, which will be referred to collectively as compressors. The invention provides improved structure for mounting the impeller blades on the fan hub, and also provides improved stability with respect to aerodynamic stalling.

One important object of the invention is to provide structure by which the pitch angle of the mounted impeller blades can be adjusted conveniently, while insuring reliable locking of the blades in adjusted position. The invention further permits convenient and rapid disassembly of one or more blades individually, as for inspection or replacement. That aspect of the invention is more fully described and is claimed in the above identified parent application.

Another aspect of the invention, to which the present application is particularly directed, improves fan operation throughout a wide range of blade adjustment by reducing the tendency to abrupt aerodynamic stalling. It is well known that axial compressors are subject to instability as the air flow is throttled, typically shifting abruptly to a stalled condition with a significant drop in output pressure. That tendency varies in severity as the blade pitch angle is changed, ordinarily becoming more serious as the openings between blades are closed down.

The present invention increases the range of stable operation by providing one or more paths for backward gas flow between the pressure side and the suction side of individual blades. Such paths are typically provided via apertures in the blades, a single aperture near the blade tip being normally sufficient. One or more blades may be so apertured. When a plurality of blades are apertured, they are preferably selected symmetrically with respect to the blower axis. Provision of a small number of such paths for backfiow, typically from one to six, reduces the total pressure produced by the fan only slightly, and typically has no measurable effect on the normal flow. The resulting increase in the range of stable operation, although advantageous in any compressor, is especially valuable in combination with an impeller having adjustable blades.

A full understanding of the invention, and of its further objects and advantages, will be had from the following description of certain illustrative manners of carrying it out, which description is to be read with relation to the accompanying drawings, in which 3,664,757 Patented May 23, 1972 FIG. 1 is a transverse section of an illustrative compressor impeller embodying the invention, as seen from the pressure side; and

FIG. 2 is a schematic graph illustrating the eifect of one aspect of the invention on the stall characteristic of a typical compressor.

In FIG. 1 an axial flow compressor is shown in transverse section, as seen from the pressure side, with impeller 20 mounted on the shaft 22 and enclosed in the cylindrical housing 24. Shaft 22 is journaled on the axis 21 and is driven in conventional manner, not explicitly shown. Impeller 20 comprises the hub 26 on which the blades 30 are mounted by mechanism described and claimed in the above identified parent application. The removable plate 36 provides access to the blades one at a time through the aperture 34 in housing 24.

In accordance with the present aspect of the invention, one or more selected blades are apertured, the apertures being most effective when near the blade tip and closer to the leading edge than to the trailing edge of the blade, as illustratively shown at 32, 32a and 32b. When the compressor is operated under design conditions, the apertures or ports 32 are found to have only negligible effect on the total pressure that is produced, typically lowering that pressure by 1 or 2%. There is ordinarily no measurable eifect upon the total flow. However, as the flow is reduced by throttling of the output, the abrupt aerodynamic stalling that is characteristic of axial compressors under such conditions is greatly reduced or entirely eliminated.

In FIG. 2 the dashed lines are a schematic plot of total pressure rise as a function of flow for constant speed operation of a typical conventional single stage compressor. As the weight flow is reduced by throttling from the normal operating region A to the critical region B below the peak pressure, the total pressure tends to decrease abruptly, as at C, to a lowered value, shown by the curve D. If the flow is then again increased, the compressor recovers abruptly, but on the line B, which corresponds to a higher flow rate than that at which the decrease C occurred. Hence there is a hysteresis region of unstable operation, represented in the present illustrative diagram by the area enclosed by lines B, C, D and E. As is well known, such instability can lead to serious difiiculties and even dangers, depending upon the function of the particular compressor.

In contrast, an impeller provided with antistall ports in accordance with the present invention typically produces the stable operation represented by the solid line F in FIG. 2. The total pressure rise produced in the normal operating region A is only slightly reduced. That reduction increases gradually as the flow is throttled, but typically merges smoothly and without any sharp break into curve D, the normal operating characteristic for low flow. As the flow is increased, the pressure recovers along the same line F, without hysteresis.

The phenomenon of abrupt stall is not well understood, and no successful analytical treatment has been proposed. Therefore no detailed analysis can be given of the described action of the antistall ports 32 of the present invention. However, that action is believed to be due, at least in part, to the production of a local stall condition adjacent each port early in the throttling phase. Such local stall regions may then spread gradually to affect the entire fan, eliminating the instability and accompanying hys teresis that are normally encountered.

A remarkable feature of the described antistall apertures is that only one such aperture is suflicient to extend the region of stable operation very significantly, and may produce complete stability as shown in FIG. 2. When several apertures are used, they have been found most ef fective when located in blades that are angularly spaced in a symmetrical pattern. Such arrangement probably tends to stimulate the normal stall patterns in the blading, which usually occur symmetrically with respect to the axis. The number of blades per row is usually selected as a prime number, which is necessarily odd. FIG. 1 illustrates an impeller with 13 blades of which three are apertured, forming two blade groups that are arranged symmetrically. Although apertures 32 are shown typically as holes within the blade boundaries, the term aperture is intended to include openings that intersect the blade edge.

We claim: 1. In a rotary, axial flow gas compressor that includes a circular array of generally radial blades rotatably mounted within a tubular coaxial housing and subject to variable mass flow due to throttling of the flow; means for reducing instability of operation due to such throttling, said means comprising apertures formed in respective selected blades of the array adjacent the blade tips and forming paths for gas flow between the pressure side and the suction side of the blade, said selected blades forming a plurality of blade groups alternating with groups of unapertured blades, the apertured and unapertured blades all having the same general configuration, and the total number of apertured blades being less than half the total number of blades in the array.

2. Stability control means as defined in claim 1, and in which the total number of blades in the array is an odd numher,

and said apertured blades consist of one group containing only two apertured blades on one side of the array and a second group containing a single apertured blade symmetrically placed on the other side of the array.

3. Stability control means as defined in claim 1, and in which the apertured blades are apertured only adjacent the blade tip and closer to the leading edge than to the trailing edge of the blade.

References Cited UNITED STATES PATENTS 1,066,988 7/1913 Boutwell 4l623l 469,511 2/1892 Anderson 416-231 1,961,114 5/1934 Tully et a1. 416231 2,003,073 5/1935 Faber 416--231 2,161,182 6/1939 Massey 4l623l 2,597,510 5/ 1952 McBride 4l5Dig. 1

CORNELIUS J. HUSAR, Primary Examiner US. Cl. X.R. 

